1. Field of the Invention
The present invention relates to image scanners, and more particularly to improved Y-positioning of the scan window to achieve finer accuracy than the motor's step size. Improved repositioning is accomplished by synchronizing the center of the exposure time with the time the sensor array is over the center of the document line.
2. Related Art
Conventional scanners move the carriage at a single speed and use line dropping and duplication to obtain various resolutions. In present scanning systems the starting and restarting of scanning after a buffer full condition, for instance, is always done at a step motor's step boundary.
Once a scanner determines that scanning should halt due to a buffer full condition the stepper motor must be stopped and be restarted. However, because the carriage must decelerate for several motor steps, the stepper motor must be reversed to reposition the carriage. The restart of scanning is conventionally done at a motor step boundary. Hence, in conventional scanners, different scanning resolutions are achieved by selectively ignoring scanned lines. For instance, in a scanner with a 300 dot per inch (dpi) stepper motor speed, 200 dpi resolution can be achieved by only putting 2 out of 3 scanned lines into buffer memory.
In conventional scanners the granularity of the CCD's exposure time can cause up to one line of error in resolution of a paused scan, because only positional information is used to restart the scanner.